The present invention refers to an apparatus for assessing a silicon dioxide content of a phosphoric acid bath for etching a silicon nitride and to a system for etching silicon nitride.
Usually, in the manufacture of semiconductor integrated circuits, hot phosphoric acid baths are used for wet-etching of silicon nitride layers. Silicon nitride is generally used as a masking layer or as an insulator layer. When it becomes necessary to remove or to pattern the silicon nitride layer, normally, wet etching with hot phosphoric acid (HOT PHOS) is used. The chemical reaction that takes place is as follows:                     Si        3            ⁢              N        4              +          6      ⁢              H        2            ⁢      O        ⁢      xe2x80x83    ⁢      →                  H        3            ⁢              PO        4              ⁢      xe2x80x83    ⁢            3      ⁢              SiO        2              +          4      ⁢              NH        3            
Since the etching rate of silicon nitride (Si3N4) is more than ten times higher than the etching rate of silicon dioxide (SiO2), silicon nitride is selectively etched. However, many factors influence the etching rate of both silicon nitride and silicon dioxide. They include: (i) a bath temperature which is held at a constant temperature (ii) a water content of the bath, which determines the overall etching rate of the bath, and, in particular, (iii) an SiO2 content of the bath which strongly influences the selectivity.
Due to water evaporation and the reaction as indicated above, the concentrations of water as well as of silicon dioxide are continuously changing during the etching process. Therefore, for a well-controlled process, at least these two parameters should be controlled, too.
In particular, the SiO2 content of a phosphoric acid etching bath should be in a certain range. More specifically, a minimum SiO2 content is required in order to maintain the selectivity of the etching of Si3N4 with respect to SiO2. On the other hand, there is only a limited amount of SiO2, which can be dissolved in the etching solution. More concretely, the higher the SiO2 content in the solution, the higher the expected amount of non-dissolved SiO2. However, the particles could have a bad influence on the wafer surface or, even worse, could block filters of the etching equipment. Moreover, SiO2 can even be existent as a colloid, which also negatively influences the characteristics of the etching bath.
However, it is difficult to measure the SiO2 content of a phosphoric acid bath, especially with an in-situ method, which is necessary for the online control of the bath.
U.S. Pat. No. 6,207,068 discloses a silicon nitride etch bath containing a circulation line having a silicon dioxide condensing system formed of a heat exchanger and a secondary filter. More specifically, part of the phosphoric acid bath is cooled whereby the solubility of silicon dioxide is reduced and the precipitated silicon dioxide particles are extracted by the secondary filter. As a result, the overall silicon dioxide content of the phosphoric acid bath is reduced by a small amount. However, the system does not allow a measurement of the silicon dioxide content of the phosphoric acid bath. Accordingly, the silicon dioxide content is only reduced without any control thereof.
It is accordingly an object of the invention to provide an apparatus for assessing the silicon dioxide content of a phosphoric acid bath as well as a system for etching silicon nitride, which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for assessing a silicon dioxide content of a phosphoric acid bath for etching silicon nitride. The apparatus contains a sensor for measuring a concentration of NH3 in the phosphoric acid bath, a storage unit for storing data defining a relationship between a content of the silicon dioxide and the concentration of NH3, and a device for calculating the content of the silicon dioxide on a basis of the concentration of NH3 measured and the data stored in the storage unit. The device is connected to the storage unit and coupled to the sensor.
According to the present invention, the above object is achieved by an apparatus for assessing the content of silicon dioxide of a phosphoric acid bath for etching silicon nitride. The apparatus contains a sensor for measuring the concentration of NH3 of the phosphoric acid bath, a recording unit for storing data that define a relationship between the content of silicon dioxide and concentration of NH3, and a device for calculating the content of silicon dioxide on the basis of the measured concentration of NH3 and the stored data.
Moreover, the present invention provides a system for etching silicon nitride and contains a process tank filled with a phosphoric acid bath, the apparatus for assessing the content of silicon dioxide of the phosphoric acid bath as defined above, and a monitoring system for monitoring the content of silicon dioxide.
As defined in the above equation, NH3 is also a reaction product of the etching reaction. The influence of the NH3 amount on the etching rate or selectivity has not yet been clarified so far. However, as the inventors of the present invention surprisingly found out, the concentration of NH3 is directly related with the SiO2 content of the solution. In particular, it has experimentally proven that both the SiO2 and the NH3 content increase proportional to the number of processed wafers, i.e. the amount of etched Si3N4.
As a consequence, it is possible to monitor the SiO2 content by determining the NH3 content of the solution. Since, in comparison to the detection of the silicon dioxide content, it is easier to assess the NH3 content of a solution, for example by optical (in particular infrared) absorption measurement or titration, the SiO2 content can easily be measured indirectly.
In particular, the apparatus for assessing the silicon dioxide content of the present invention contains a storage unit for storing data, which define a relationship between the content of silicon dioxide and concentration of NH3. The data can be, for example, obtained from calibration measurements, which have to be conducted for each etching system in given time intervals.
The system for etching silicon nitride of the present invention contains a monitoring system for monitoring the content of silicon dioxide. The monitoring system is specifically adapted to decide whether the content of silicon dioxide of the bath is higher than a first predetermined value and lower than a second predetermined value.
In particular, if the content of silicon dioxide of the bath is higher than the second predetermined value, SiO2 particles are likely to precipitate. Accordingly, the content of silicon dioxide of the bath must be lowered, for example by simply exchanging the phosphoric acid bath.
On the other hand, if the content of silicon dioxide of the bath is lower than the first predetermined value, the high etching selectivity between Si3N4 and SiO2 is no longer given, and it is necessary to elevate the SiO2 content of the bath. Since the SiO2 content can easily be determined by the above mentioned method, the time at which the bath conditioning is to be finished can as well be easily determined.
Commonly used systems for etching silicon nitride also contain a circulation line for circulating part of the phosphoric acid bath. Accordingly, the sensor for assessing the NH3 content can either be placed in the process tank or it can be placed in the circulation line. Alternatively, the sensor can be disposed in a bypass line connected to the circulation line.
The position of the sensor for assessing the NH3 content can be chosen in dependence from the specific detection method used. As a suitable detection method any method as generally known in the art for assessing the NH3 content of a liquid can be used. In particular, optical absorption using infrared light or titration can be used.
Accordingly, the present invention provides the now described advantages. The present invention provides an apparatus for measuring the SiO2 content of a phosphoric acid bath used for etching silicon nitride layers. Thereby, the age of the phosphoric acid bath can exactly be determined. Since there are commercially available systems for measuring the NH3 concentration, the apparatus of the present invention is very easy to handle and cheap to build. Accordingly, the silicon dioxide of the etching bath can be in-situ monitored whereby a high selectivity of the etching process is given and, at the same time, the precipitation of silicon dioxide particles is prevented. In particular, in the system for etching silicon nitride of the present invention, the monitoring system will preferably decide whether the silicon dioxide content is too low or too high. Based on this result, either the silicon dioxide content is to be raised, or it is to be lowered, for example by exchanging the etching bath. Accordingly, an online process control of the etching parameters is possible.
As a consequence, the phosphoric acid etching bath can more effectively be used without the danger of filter blocking and precipitation of silicon dioxide, whereby the cost will be largely reduced. In particular, by exactly determining the bath lifetime the consumption of phosphoric acid can be reduced.
Since the bath age can be determined exactly, the lifetime of the filters can be prolonged or the filters can even be removed completely. Accordingly, the cost can further be reduced.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for assessing a silicon dioxide content, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.